1. Field of the Invention
The present invention relates generally to thermoforming of a plastic sheet. More specifically, the invention relates to a chilled air pressure box for thermoforming a plastic sheet and a method of thermoforming a plastic sheet.
2. Background Art
Appearance is often an important consideration in manufacturing parts with exposed surfaces, including but not limited to exterior and interior vehicle trim components. For this reason, original equipment manufactures (OEM) or automotive part suppliers may specify requirements regarding visible surfaces of trim components, sometimes referred to as Class “A” surfaces, including gloss targets and acceptable gloss ranges. For instance, an automotive exterior component such as a bumper fascia or a body side molding may have a Class “A” high gloss target value of 85 as measured with a 20 degree BYK Gardner gloss meter. However, the acceptable gloss range may be 80 to 90. An automotive interior component such as an instrument panel may have, for example, a Class “A” low gloss target of 2 with an acceptable gloss range of 1.5 to 2.5. This lower gloss target and more narrow gloss range for an instrument panel may in part be due to veiling glare requirements. Thus, gloss targets and acceptable ranges of gloss may vary considerably depending on the particular part and the desired visual effect.
Typical techniques for manufacturing these trim components may often be accomplished by injection molding the component where the color and gloss may be imparted either by painting the part after it has been injection molded or by directly molding in the color and the gloss. This latter process is often referred to as molded in color (MIC). MIC is either done by using a color concentrate masterbatch, which is blended into a plastic resin mixture during the injection molding process, or pre-coloring the plastic resin mixture prior to the resin being introduced to the injection molding machine. Both painting and MIC production techniques are proven and function well.
However, there are a number of negatives associated with each of these processes. For instance, injection molding requires large steel tools to be cast and machined. This is expensive and often requires long lead times. In some cases, for example, a large component such as a bumper fascia may require 36 to 52 weeks to produce production tooling.
The painting process may have a long cycle time where the parts may need to be post baked for 30 minutes or longer at an elevated temperature, such as 250° F. Additionally, many of these trim components may be injection molded from a polyolefin material, such as TPO, PP or a TPE, where an expensive surface treatment prior to painting, such as corona treatment, flame treatment or application of a chlorinated polyolefin adpro, may be required for proper paint adhesion. High scrap rates due to surface defects, low film builds and poor adhesion of the paint to the part may also occur.
Parts produced by a MIC process may lack the added surface protection provided by the paint coating used on painted components, and may therefore be more susceptible to chemical attack from fuels and other fluids, UV fading, scratches and other surface marring. Moreover, MIC parts may have a propensity to stress whitening, which is a white surface blemish resulting from handling or impact stresses.
Presently, new technologies are developing that may minimize higher cost, longer lead times and other short comings associated with injection molding and decoration of parts by either MIC or painting. These technologies involve the use of a thermoformable plastic sheet comprising a thermoplastic layer and an opposite paint film layer. The paint film layer may have a color and an initial gloss that meets the Class “A” specifications established by an OEM or others. Additionally, the paint film layer may optionally include a dry paint layer covered by a clear coat layer such as polyvinylidene fluoride PVDF, which may be appropriate for not only meeting the Class “A” craftsmanship requirements of a component but other performance requirements such as chemical resistence, fade, scratch and mar.
While the technology for thermoforming a plastic sheet has become well established, there remains the problem that current methods of thermoforming when used to thermoform a laminate plastic sheet comprising a thermoplastic layer and a paint film layer may adversely affect the gloss of the paint film layer. This affect on gloss may be due to hazing, which may result within the paint film layer if some critical temperature is reached. This problem may be further compounded if the temperature required for thermoforming the thermoplastic layer exceeds a critical hazing temperature of the paint film layer. Current methods of thermoforming do not adequately provide active cooling techniques that allow the paint film layer to be maintained below a critical temperature as thermal conduction from the higher temperature thermoplastic layer occurs.
In view of the above, it is apparent that there exist a need for a cooling apparatus, together with a method of thermoforming a plastic sheet having a paint film layer, which preserves the gloss finish of the paint film layer within an acceptable range, especially under high volume manufacturing conditions.